1. Field of the Invention
The present invention relates to a voltage controlled oscillator which outputs an oscillation signal of a desired oscillation frequency, an operation current adjusting device which controls the voltage controlled oscillator, and an operation current adjusting method of the voltage controlled oscillator.
2. Background Art
Conventionally, for example, in a mobile communication terminal whose market has been rapidly expanding, a voltage controlled oscillator (VCO) needs to be provided for each terminal.
As characteristics of the voltage controlled oscillator, the phase noise characteristic which is a parameter indicating a degree of oscillation purity is very important along with the oscillation frequency and the frequency variable range.
In the case of a common voltage controlled oscillator, a change in the phase noise characteristic with respect to the oscillation frequency is large. This is because loss in a resonator is different for each oscillation frequency. Further, the phase noise characteristic is also different depending upon manufacturing conditions. This is considered due to the fact that loss in a resonator varies depending upon manufacturing conditions, and that characteristics of an amplifier circuit and a power supply circuit of the voltage controlled oscillator also vary depending upon manufacturing conditions, or the like.
Here, as the above described voltage controlled oscillator, there is for example an oscillator provided with an amplifier circuit which includes an inductor and a variable capacitance element and outputs an oscillation signal of an oscillation frequency corresponding to an oscillation frequency control voltage supplied to the variable capacitance element, and with a power supply circuit which supplies an operation current to the amplifier circuit.
In this voltage controlled oscillator, the change in the phase noise characteristic with respect to the oscillation frequency is mitigated by controlling the power supply circuit simultaneously with the oscillation frequency control voltage. That is, the voltage controlled oscillator is designed to be compensated in such a manner that the change in the phase noise characteristic is mitigated by directly measuring the loss property of the resonator and the phase noise characteristic in advance (see, for example, Japanese Patent Laid-Open Publication No. 2001-313527).
However, in the above described conventional voltage controlled oscillator, there is a case where the loss property of the resonator is made different by variations in manufacturing each voltage controlled oscillator, or by use conditions such as ambient temperature and the like. In this case, there is a problem that the phase noise characteristic is eventually deteriorated, even when a desired compensation design is performed on the basis of the loss property of the resonator and the phase noise characteristic which were measured in advance.
In addition, a predetermined external device is required for measuring the above described phase noise characteristic. Therefore, there is for example a problem that it is difficult to automatically adjust the phase noise characteristic by a circuit configuration in a semiconductor integrated circuit including the voltage controlled oscillator.